This invention relates in general to electrophotography and more specifically, to an improved electrophotographic imaging member with vapor deposited generator layer and improved adhesive layer and process for using the imaging member.
In the art of electrophotography an electrophotographic plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
Flexible electrophotographic imaging member belts are usually multilayered photoreceptors that comprise a substrate, an electrically conductive layer, an optional hole blocking layer, an adhesive layer, a charge generating layer, a charge transport layer and, in some embodiments, an anti-curl backing layer. One type of multilayered photoreceptor comprises a layer of finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. U.S. Pat. No. 4,265,990 discloses a layered photoreceptor having separate charge generating (photogenerating) and charge transport layers. The charge generating layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer.
One type of popular photoreceptor is a flexible belt photoreceptor which comprises a thin metal coating ground layer over a flexible polymeric substrate support and two electrically operative layers, including a charge generating layer and a charge transport layer. The electrically conductive ground layer may be formed, for example, on a flexible biaxially oriented substrate by a suitable coating technique, such as vacuum deposition of metals.
After formation of an electrically conductive ground plane, a hole blocking layer may be applied thereto. Where the metallic ground plane is metallic, the hole blocking layer may comprise polyvinylbutyral; organosilanes; epoxy resins; polyesters; polyamides; polyurethanes; pyroxyline vinylidene chloride resin; silicone resins; fluorocarbon resins and the like containing an organo metallic salt; and nitrogen containing siloxanes or nitrogen containing titanium compounds and the like.
In some cases, an intermediate layer between the charge blocking layer and the adjacent generator layer may be used in the photoreceptor to improve adhesion or to act as an electrical barrier layer. Typical adhesive layers disclosed, for example, in U.S. Pat. No. 4,780,385 include film-forming polymers such as polyester, polyvinylbutyral, polyvinylpyrolidone, polyurethane, polycarbonates, polymethylmethacrylate, mixtures thereof, and the like.
The photogenerating layer utilized in multilayered photoreceptors include, for example, inorganic photoconductive particles such as amorphous selenium, trigonal selenium, and selenium alloys selected from the group consisting of selenium-tellurium, selenium-tellurium-arsenic, selenium arsenide and mixtures thereof, and organic photoconductive particles including various phthalocyanine pigments such as the X-form of metal free phthalocyanine, metal phthalocyanines such as vanadyl phthalocyanine and copper phthalocyanine, quinacidones available from DuPont under the tradename Monastral Red, Monastral violet and Monastral Red Y, Vat orange 1 and Vat Orange 3 trade names for dibromo anthanthrone pigments, benzimidazole perylene, substituted 2,4-diaminotriazines, polynuclear aromatic quinones available from Allied Chemical Corporation under the tradename Indofast Double Scarlet, Indofast Violet Lake B, Indofast Brilliant Scarlet and Indofast Orange, and the like dispersed in a film forming polymeric binder. Selenium, selenium alloy, benzimidazole perylene, and the like and mixtures thereof may be formed as a continuous, homogeneous photogenerating layer. Benzimidazole perylene compositions are well known and described, for example in U.S. Pat. No. 4,587,189. Other suitable photogenerating materials known in the art can be utilized, if desired. Charge generating binder layers can be used. These binder layers comprise photoconductive particles dispersed in a binder resin such as vanadyl phthalocyanine, metal free phthalocyanine, benzimidazole perylene, amorphous selenium, trigonal selenium, selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic, selenium arsenide, and the like and mixtures thereof in a selected polymer matrix.
Although excellent images may be obtained with multilayered photoreceptors, it has also been found that for certain specific combinations of materials in the different layers, adhesion of the various layers under certain manufacturing conditions can fail and result in delamination of the layers during or after fabrication. Photoreceptor life can be shortened if the photoreceptor is extensively image cycled over small diameter rollers. Also, during extensive cycling, many belts exhibit undesirable dark decay and cycle down characteristics. The expression "dark decay" as employed herein is defined as the loss of applied voltage from the photoreceptor in the absence of light exposure. "Cycle down", as utilized herein, is defined as the increase in dark decay with increasing charge/erase cycles of the photoreceptor.
A typical multilayered photoreceptor exhibiting dark decay and cycle down under extensive cycling utilizes a charge generating layer containing trigonal selenium particles dispersed in a film-forming binder. It has also been found that multilayered photoreceptors containing charge generating layers utilizing trigonal selenium particles are relatively insensitive to visible laser diode exposure systems.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during extended cycling. Moreover, complex, highly sophisticated, duplicating and printing systems operating at very high speeds have placed stringent requirements including narrow operating limits on photoreceptors. For example, the layers of many modern photoconductive imaging members must be highly flexible, adhere well to each other, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles.
An encouraging advance in electrophotographic imaging which has emerged in recent years is the successful fabrication of a flexible imaging member which exhibits a nearly ideal capacitive charging characteristic, outstanding photosensitivity, low electrical potential dark decay, and long term electrical cyclic stability. This imaging member employed in belt form usually comprises a substrate, a conductive layer, a solution coated hole blocking layer, a solution coated adhesive layer, a thin vacuum sublimation deposited charge generating layer of pure organic pigment, a solution coated charge transport layer, a solution coated anti-curl layer, and an optional overcoating layer. This type of photoreceptor is described, for example, in U.S. Pat. No. 4,587,189 in which a benzimidazole perylene charge generating layer is formed by vacuum sublimation. This multilayered belt imaging member provides excellent electrical properties and extended life. However, it has been found that this photoreceptor is susceptible to the formation of cracks in the charge generating layer. Since these cracks have an appearance similar to cracks found in dried mud flats, they are often referred to as "mud cracks". These observed mud cracks in the charge generating layer comprise a two dimensional network of cracks. Mud cracking is believed to be the result of built in internal strain due to the vacuum sublimation deposition process and subsequent solvent penetration through the thin charge generating layer. The penetrating solvent dissolves the adhesive layer underneath the generating layer during application of the charge transport layer coating solution. Crack formation in the charge generating layer seriously impacts the versatility of this type of photoreceptor and can reduce the practical value of the photoreceptor. Cracks in charge generating layers not only print out as defects in the final copy, but may also act as strain concentration centers which propagate the cracks into the other electrically operative layer, i.e., the charge transport layer, during dynamic belt cycling in copiers, printers and duplicators. Omission of the solution coated adhesive layer from the flexible electrophotographic imaging member material package and vacuum deposition of the charge generating layer directly on the hole blocking layer has been found to successfully eliminate the charge generating layer mud cracking problem altogether and provide a simplified imaging material structure. Unfortunately, the resulting electrophotographic belt imaging member spontaneously delaminates after only a few hundred cycles of machine testing. The observed premature mechanical failure of the imaging member belt appears to be due to the stress/strain fatiguing conditions induced by flexing over the belt support rollers as well as mechanical interaction with various machine subsystems during imaging, development, image transfer, and belt cleaning.
Although this discussion has focused primarily on photoreceptors with benzimidazole perylene charge generating layers, the appearance of mud cracking is, in fact, a problem photoreceptors described above which utilize a vacuum sublimation deposited charge generation layer.
While the above-described imaging member exhibits desirable electrical characteristics, there is an urgent need to resolve the cracking problem in order to achieve an imaging member capable of forming high quality prints under extended image cycling conditions. It is also important that any solution employed to solve the charge generating layer mud cracking problem does not produce any deleterious electrical or mechanical integrity effects in the modified device.
Thus, there is a continuing need for an electrophotographic imaging member having improved resistance to mud crack formation in the charge generating layer.